Investigation of thermoluminescence and photoluminescence properties of Tb³⁺, Eu³⁺, and Dy³⁺ doped NaYF₄ phosphors for dosimetric applications

Hexagonal phase sodium yttrium fluoride activated with lanthanide ions; Tb³⁺, Eu³⁺ and Dy³⁺ doped NaYF₄ phosphors were synthesized using a simplistic hydrothermal method. The photoluminescence studies demonstrated green, red and blue emission lines corresponding to ⁵D₄ → ⁷F_J (J = 6, 5, 4, 3), ⁵D₀ → ⁷F_J (J = 1, 2 and 4) and ⁴F_9/2 to ⁶H_J (J = 15/2 and 13/2) transitions, which are characteristic of Tb³⁺, Eu³⁺ and Dy³⁺ ions, respectively. The as-synthesized samples were subjected to annealing at varying temperatures from 500 °C to 800 °C primarily for the optimization of the thermoluminescence glow curve. Meanwhile, we studied the influence of thermal annealing treatment on the crystal phase, morphological features, and photoluminescence properties of the phosphors. The spherical-like morphology of NaYF₄:Tb³⁺ phosphor changed to micro block-like structures and the hexagonal phase of NaYF₄ transforms into a cubic phase at a higher annealing temperature of ∼800 °C. The photoluminescence emission intensity also varied at different annealing temperatures. The systematic study using different dopants and annealing temperatures was carried out to accomplish efficient thermoluminescence (TL) properties. The most suitable TL dosimetric glow peak was attained for NaYF₄:0.5%Tb³⁺ phosphor, which was annealed at a temperature of 800 °C, located at 194 °C. The NaYF₄:Tb³⁺ phosphor exhibited TL response fairly linear in the dose range from 1 kGy to 25 kGy of gamma radiation. The phosphor showed TL response at higher doses, which stipulates that the NaYF₄:Tb³⁺ is reasonably well suitable for high dose measurements and respective applications. The phosphor exhibited negligible fading and good reproducibility features. Trap-level analysis and experimental determination of activation energy were performed using the T_m-T_stop technique and initial rise method (IRM). The trapping parameters of the TL glow curve, such as activation energy (E), order of kinetics (b), and frequency factor (s) were estimated by Chen's glow peak shape (PS) method and glow curve deconvolution (GCD) method. The trapping parameters obtained using the IRM, PS and GCD methods are in good accordance with each other. Henceforth, along with efficient photoluminescence properties, the NaYF₄:Tb³⁺ phosphor exhibited favorable thermoluminescence dosimetric properties. Consequently, this study provides new opportunities for utilizing these phosphors in the area of radiation dosimetry applications such as environmental and food monitoring, space dosimetry etc.